Electrolysis cells subdivided by a diaphragm into two electrode compartments are frequently used in electrochemical processes; a common feature of such cells is the need of keeping one electrode in contact with the separator, while the other electrode acts as a rigid support for the separator itself. In the specific case in which the separator is a diaphragm or an ion-exchange membrane, the above cell design is widely applied in chlor-alkali electrolysis, which is known to represent one of the few electrochemical processes of widespread industrial application. In the case of the chlor-alkali process, the most common design provides the anodic compartment to contain a rigid anode consisting of a titanium punched sheet or expanded sheet or mesh coated with a superficial electrocatalytic film for chlorine evolution comprising noble metal oxides; this compartment is fed with a concentrated solution of an alkali chloride, usually sodium chloride.
The structure of the cathodic compartment may encompass different types of mechanical arrangement.
In the version described for instance in U.S. Pat. No. 5,225,060 the cathodic compartment, fed with a caustic soda solution, contains a rigid cathode consisting of a nickel punched sheet or expanded sheet or mesh, optionally provided with an electrocatalytic film for hydrogen evolution. An ion-exchange membrane is installed between the anode and the cathode and being the pressure in the cathodic compartment normally higher than in the anodic compartment, the membrane is pushed by pressure differential against the anode. Since both the anode and the cathode consist of rigid parallel planar structures, it is necessary to maintain a certain gap between the two facing surfaces in order to avoid that the inescapable deviations from the ideal parallelism bring the anode and cathode surfaces in simultaneous contact with the membrane: in such conditions, the contact pressure would not be controllable and the membrane could be heavily damaged. The need of maintaining a certain gap between the anodic and the cathodic surface, indicatively of 2-3 mm, entails a cell voltage penalty associated with the ohmic drop generated by the electric current crossing the liquid phase between cathode and membrane: since cell voltage is directly proportional to energy consumption per unit weight of product chlorine or caustic soda, it follows that the overall process economics are sensibly disfavoured.
To overcome this inconvenient, membrane chlor-alkali cell design was improved in time giving rise to the development of cathodic structures capable of bringing the cathode surface in contact with the membrane under moderate and predefined contact pressures.
A first family of cells, disclosed for instance in U.S. Pat. No. 5,254,233 and U.S. Pat. No. 5,360,526, provides the cathode in form of punched sheet or expanded sheet or mesh to be secured to elastic supports consisting of springs of various design, fixed in their turn to a planar current distributor or directly to a cell wall: the springs, upon assembling the cell, compress the cathode against the membrane at a pressure only dependent on their elastic behaviour and on the deviations from the anode-to-current distributor or anode-to-cell-wall distance associated with the construction tolerances. The drawback of this constructive solution is given by the fact that the sheet or mesh must present a certain stiffness in order to allow springs working in a correct fashion: it follows that the uniformity of cathode-membrane contact requires a perfect planarity of both surfaces, of the membrane-supporting anode and of the cathode pressed against the membrane by the springs. Such planarity is not reasonably obtainable with big sized anodes and cathodes suitable for being installed in cells of high productive capacity normally employed in modern industrial plants. Hence, even though the cathode surface is brought into contact with the membrane surface, there still exist areas in which the two surfaces remain spaced apart, with the consequence of an inhomogeneous current distribution and of an energy consumption above the expected values.
A second family of cells, disclosed for instance in U.S. Pat. No. 4,444,632 and U.S. Pat. No. 5,599,430, is directed to overcome the inconveniences associated with the stiffness of the cathodic sheet or mesh of U.S. Pat. No. 5,254,233 and U.S. Pat. No. 5,360,526 by utilising as the cathode a thin punched sheet or expanded sheet or mesh provided with high flexibility and therefore highly adaptable when pressed against the surface of the anode-supported membrane, even when the rigid anode presents a non perfectly planar surface profile. The highly flexible structure of the cathode entails as a first consequence that springs disclosed in U.S. Pat. No. 5,254,233 and U.S. Pat. No. 5,360,526 can't be usefully employed anymore since their mechanical action would not be uniformly transferred to the whole cathodic surface. For this reason, in the cited documents there is disclosed the use of planar elastic layers, somehow acting as distributed springs. These layers consist of planar elements formed by weaving of nickel wires and subsequently corrugated: such elements are juxtaposed in a number of at least two to form elastic structures, preferably with crossed corrugations so as to minimise interpenetration, and may be enclosed between two further cloths of planar wires. One alternative construction provides the planar layer to consist of at least two juxtaposed elements formed by interconnected coils of wire. In known cell designs the above disclosed elastic layers form part of a cathodic package normally comprising a rigid current distributor made of a nickel punched sheet or expanded sheet or mesh of adequate thickness, the elastic layer and a thin and highly flexible nickel punched sheet or expanded sheet or mesh optionally provided with an electrocatalytic film for hydrogen evolution. Upon assembling the cell by joining the anodic and the cathodic compartment, such elastic layer is partially compressed, thereby transferring a pressure to the thin and flexible cathode which in its turn is pressed against the rigid anode-supported membrane: since the elastic layer contacts the cathode in a multiplicity of points, the compression exerted on the membrane is substantially distributed and moreover the cathode flexibility ensures that the latter is completely adapted to the membrane profile. The aim of this rather complex layered structure is to ensure an elastic behaviour characterised by linearity and by a modest angular coefficient of the relationship of the pressure exerted in a partially compressed state to the corresponding compression: such features are required since only in this way it is possible to ensure a good homogeneity of cathode-to-membrane pressure at moderate values, so that integrity of the membrane is preserved. Nevertheless, this objective is only partially achieved with the above elastic structures, essentially due to the impossibility of completely avoiding interpenetration between the various wire elements, with the consequence of an inevitable local variability of pressure exerted on the cathode-membrane assembly, attaining peak levels that can impair a correct preservation of the membrane. Furthermore, structures of such a complexity bear high production costs and a difficulty of installation scarcely compatible with the economics of an industrial process. These inconveniences are only partially overcome by the teaching of WO 03/048422, wherein corrugated planar elements juxtaposed with crossed corrugations are enclosed in a flattened stocking-type structure made of woven wires: the stocking containing the two corrugated elements in its interior constitutes in fact an easier assembly to handle and install in the cell. However, the problems of high cost and of a higher than desirable angular coefficient of the pressure to compression relationship remain unsolved.
The above prior art analysis shows the need of developing an elastic current collector suitable for being installed in an electrolysis cell characterised by a modest angular coefficient of the pressure to compression relationship over a wide range of compression values, by simplicity of installation and by a moderate cost.